Heretofore, a brushless DC motor has been used as a motor for rotating and driving a magnetic disk device. The motor of this type has been called a spindle motor. A well-known one has a structure having a stator provided with stator coils to which an exciting current is supplied, a rotor provided with a rotor magnet for obtaining a rotating power by the electromagnetic interaction with the stator coils, and a position detecting sensor for detecting the rotating position of the rotor magnet. In motors of this type, the rotating angular position of the rotor magnet is detected by the position detecting sensor, and based on the detected signal from the position detecting sensor, the exciting current to be supplied to the stator coil is switched off. A hall element is used as the position detecting sensor.
In recent years, however, to construct the motor in small-sizes or avoid the deterioration of characteristics of the sensor, a so-called sensorless DC motor which detects the position of a rotor magnet by utilizing induced voltage generated in a coil through which an exciting current does not flow without using a position detecting sensor has been generalized.
Since in the sensorless motor, a back electromotive force cannot be obtained when the motor stops, the rotor is swivelled at the time of starting. For example, in a three-phase coil spindle motor, a stepping step of successively supplying an exciting current to the stator coil is repeated. In this stepping step, an exciting current usually in a positive direction, a cessation period, or a reverse direction is supplied to each phase.
However, in such a sensorless DC motor, the technical problems to be described below exist in the method of its starting. In the above sensorless DC motor, the position of the rotor magnet is detected by the induced voltage, but since there is no induced voltage at the time of stoppage of the motor, and furthermore, the polarity of the magnet is unknown, a signal of a predetermined pattern is formed forcibly, and the motor is started. However, depending upon the position of the rotor, poor starting occurs because of a low torque. Or a magnetic field is developed in a reverse direction, and at the time of rising during starting, the rotor may rotate in a reverse direction.
In order to avoid such inconveniences and increase starting reliability, the present applicant proposed a method of starting a novel sensorless motor, as disclosed in U.S. Pat. No. 5,235,264. This starting method includes a reverse exciting operation of reversing the current flowing direction from positive to negative or from negative to positive without including a cessation time at the time of starting the sensorless motor. When this reverse exciting operation is carried out, a large flux density changing width occurs and the dead point of starting is dissolved, and simultaneously, a high torque is developed, and starting reliability increases.
In this improved starting method, usually after a first stepping step of supplying an exciting current to a stator coil with a predetermined inner stepping pattern, an exciting current is supplied with the same inner stepping step as above in a second stepping step, and in the early period of the second stepping step, a reverse exciting operation is obtained. Accordingly, when the motor is not at all started in the first stepping step, a reverse exciting operation is carried out in the early period of the second stepping step, and the probability of starting the motor becomes very great.
However, in such a method of starting, it is only the case of a one-phase coil that a reverse exciting operation is carried out in the early period of the second stepping step. Accordingly, it is impossible to obtain a very sufficient starting torque, and the method is still required to be improved.